Warming assessment of the bottom-up Paris Agreement emissions pledges

The Guardian Headline for this study is rather alarmist:

Policies of China, Russia and Canada threaten 5C climate change, study finds  Ranking of countries’ goals shows even EU on course for more than double safe level of warming

Here is the abstract and introduction:

Link to full study.

• Yann Robiou du Pont &
• Malte Meinshausen

Nature Communications volume 9, Article number: 4810 (2018)

Abstract

Under the bottom-up architecture of the Paris Agreement, countries pledge Nationally Determined Contributions (NDCs). Current NDCs individually align, at best, with divergent concepts of equity and are collectively inconsistent with the Paris Agreement. We show that the global 2030-emissions of NDCs match the sum of each country adopting the least-stringent of five effort-sharing allocations of a well-below 2 °C-scenario. Extending such a self-interested bottom-up aggregation of equity might lead to a median 2100-warming of 2.3 °C. Tightening the warming goal of each country’s effort-sharing approach to aspirational levels of 1.1 °C and 1.3 °C could achieve the 1.5 °C and well-below 2 °C-thresholds, respectively. This new hybrid allocation reconciles the bottom-up nature of the Paris Agreement with its top-down warming thresholds and provides a temperature metric to assess NDCs. When taken as benchmark by other countries, the NDCs of India, the EU, the USA and China lead to 2.6 °C, 3.2 °C, 4 °C and over 5.1 °C warmings, respectively.

Introduction

Since the adoption of the United Nations Framework on Climate Change Convention (UNFCCC)¹ and its objective to stabilize greenhouse gas (GHG) concentrations to avoid dangerous global warming, most countries have committed to limiting GHG emissions through domestic measures or support of mitigation action abroad. Informed by literature on effort-sharing approaches, the international community has long discussed the operationalization of equity following the UNFCCC principle of Common But Differentiated Responsibilities and Respective Capabilities (CBDR-RC) to drive national emissions allocations^2,3. The failure to agree on a top–down mechanism to derive binding national emissions targets for all countries led to a bottom-up situation where countries should pledge NDCs of highest possible ambition^4,5. While the quest for a common understanding of what is a fair effort-sharing continues, rapidly falling technology costs of renewables and increasing mitigation co-benefits shift the attention away from effort-sharing considerations⁶. However, current bottom-up NDCs do not add up to a global ambition consistent with the joint temperature goals^4,7,8,9. A 5-year stocktake requires all countries to pledge enhanced actions and support⁴.

The quantification of national emissions levels consistent with both Paris Agreement’s mitigation and equity goals relies on contentious interpretations of distributive justice^2,8,10. Scientists, non-governmental organizations and government experts have suggested multiple effort-sharing approaches to derive equitable national emissions allocations^{2,8,11,12,13,14,15,16,17,18,19,20,21,22}. While not all countries use indicators that favour their equity argument in their communication¹⁰, a common definition of equity is unlikely to be adopted since countries generally tend to support interpretations of distributive justice that best serve their self-interest and justify their negotiating positions^23,24,25,26. Developed countries who committed to take the lead in reducing emissions and mobilizing finance for developing countries⁴ often submitted NDCs that do not match the concepts of equity that they publicly supported²⁷ and leave the Green Climate Fund poorly funded²⁸. Their NDCs often imply a status-quo in terms of global emissions shares²⁷, while most of the very ambitious NDCs are from smaller developing countries  (https://paris-equity-check.org/).

The UNFCCC does not specify whether its principles and the CBDR-RC refer to distinct principles or to a single operationalization of equity²⁹. A way to reconcile this ambiguity is to combine multiple dimensions of equity using weighting factors^{12,14,15,16,17,22} and per-capita income thresholds^14,16 in a single effort-sharing approach applicable to all countries. Alternatively, effort-sharing approaches can be combined in a differentiated manner where countries follow different equity principles. A recent study allocated emissions to each country using the least-stringent of two equity allocations¹¹. The global level of ambition of each equity allocation was then set by a diversity-aware leader so that the sum of all countries’ allocations matches 2 °C-consistent levels¹¹. Under that methodology, countries follow different equity approaches that are applied under different warming thresholds, which may be considered unfair by Parties to the UNFCCC.

In the present study, we use a single aspirational warming threshold that is lowered consistently until the sum of all bottom-up emissions allocations, where each country follows the least-stringent effort-sharing approach¹¹, aligns with 2 °C-consistent levels. Countries are thereby assumed to follow different effort-sharing approaches applied to a common global virtual warming threshold. Ultimately, this hybrid approach follows a bottom-up combination of equity allocations consistent with a common top–down warming threshold, which arguably reflects the hybrid nature of the Paris Agreement^10,30.

The hybrid combination of equity approaches does not constitute an equitable operationalization of the CBDR-RC principle where all countries seek to maximize absolute gain³¹ by agreeing on a common approach of equity. Rather, it reflects national preferences for relative gain³²—i.e., a country’s inclination to measure the fairness of its contribution to the global mitigation effort by looking at other countries’ efforts—rather than for domestic indicators alone. Despite claims that discussions of justice are irrelevant or dangerous in a post-Paris world, equity is fundamental for climate policy research^33,34 and scientific analyses on equitable burden-sharing can be influential on the UNFCCC processes⁵. However, the absence of agreement on an unanimous operationalization of the CBDR-RC should not be used as an excuse for inaction³ and should not leave the international community without a metric reflective of current agreements to assess the ratcheting-up process. The multiplicity of equity concepts results in a wide range of emissions allocations for countries and regions³⁵ that is sometimes used as an uncertainty range by non-experts. In a recent climate case, the District Court of The Hague ruled^36,37 that the Dutch government has to reduce 2020 emissions to at least the least-ambitious end of the range recommended by the IPCC-AR4 for the Annex I country group based on multiple equity allocations from 16 studies³⁸. The court did not pick an approach of equity and ruled for the minimum effort consistent with international treaties in light of commonly reviewed science. While the multiplication of climate litigations cases against governments³⁹ (http://climatecasechart.com/) can contribute to the ratcheting-up process, systematic court decisions that governments must follow the least-ambitious end of an equity range would be insufficient to achieve the Paris Agreement. As a first step, this paper models such a bottom-up situation where each country follows the least-ambitious of five effort-sharing approaches representative of the quantified IPCC categories³⁵. As a second step, it models the hybrid approach consistent with the current compromise where each country chooses an equitable effort-sharing approach to determine its effort but cannot directly use that approach to influence other countries’ effort.

Overall, this study presents an operationalization of the current agreement to disagree on equity concepts to achieve a common temperature goal. The hybrid approach with its bottom-up combination of equity concepts reflects the pledge-and-review architecture of the Paris Agreement and provides a metric for the ratchetting-up process. The results of this study inform on the adequacy of the emissions targets contained in current NDCs with the Paris Agreement.

HT/michel